DALLAS — An Army veteran killed by Dallas police after the sniper slayings of five officers amassed a personal arsenal at his suburban home, including bomb-making materials, bulletproof vests, rifles, ammunition and a journal of combat tactics, authorities said Friday.

The man identified as 25-year-old Micah Johnson told authorities he was upset about the fatal police shootings of two black men earlier this week and wanted to exterminate whites, "especially white officers," officials said.

A crime scene investigator looks a shot out windows after a shooting in downtown Dallas, on Friday. (Photo: LM Otero/AP via Canadian Press)

A Dallas policeman keeps watch on a street in downtown Dallas following reports of the shootings. (Photo: LM Otero/CP)

He was killed by a robot-delivered bomb after the shootings, which marked the deadliest day for U.S. law enforcement since the Sept. 11, 2001, terrorist attacks. In all, 12 officers were shot.

In Georgia, Missouri and Tennessee, authorities said gun-wielding civilians also shot officers in individual attacks that came after the black men were killed in Louisiana and Minnesota. Two officers were wounded, one critically.

President Barack Obama and Texas Gov. Greg Abbott asked for the public's prayers. In a letter posted online Friday, Abbott said "every life matters" and urged Texans to come together.

Johnson was a private first class from the Dallas suburb of Mesquite with a specialty in carpentry and masonry. He served in the Army Reserve for six years starting in 2009 and did one tour in Afghanistan from November 2013 to July 2014, the military said.

After the attack, he tried to take refuge in a parking garage and exchanged gunfire with police, Police Chief David Brown said.

The suspect described his motive during negotiations and said he acted alone and was not affiliated with any groups, Brown said.

Johnson was black. Law enforcement officials did not disclose the race of the dead officers.

The bloodshed unfolded just a few blocks from where President John F. Kennedy was slain in 1963.

The shooting began Thursday evening while hundreds of people were gathered to protest the killings in Baton Rouge, Louisiana, and suburban St. Paul, Minnesota. Brown told reporters that snipers fired "ambush-style" on the officers. Two civilians were also wounded.

This undated photo shows Micah Johnson, who was a suspect in the sniper slayings of five law enforcement officers in Dallas on July 7, 2016, during a protest over two recent fatal police shootings of black men. (Photo: Facebook via AP)

Authorities initially blamed multiple "snipers" for Thursday's attack, and at one point said three suspects were in custody. But by Thursday afternoon, all attention focused on Johnson, and state and federal officials said the entire attack appeared to be the work of a single gunman.

With the lone shooter dead, Mayor Mike Rawlings declared that the city was safe and "we can move on to healing." He said the gunman wore a protective vest and used an AR-15 rifle, a weapon similar to the one fired last month in the attack on an Orlando, Florida, nightclub that killed 49 people.

When the gunfire began, the mayor said, about 20 people in the crowd were carrying rifles and wearing protective equipment. That raised early concerns that they might have been involved. But after conducting interviews, investigators concluded all the shots came from the same attacker.

A Texas law enforcement official identified the man killed in the parking garage as Johnson. The official spoke on the condition of anonymity because he said he was not authorized to release the information.

Around midday, investigators were seen walking in and out of a home believed to be Johnson's in Mesquite.

People rally in Dallas, Texas, on Thursday to protest the deaths of Alton Sterling and Philando Castile. (Photo: Laura Buckman/AFP via Getty Images)

In Washington, the nation's top law enforcement official, Attorney General Loretta Lynch, called for calm, saying the recent violence can't be allowed to "precipitate a new normal."

Lynch said protesters concerned about killings by police should not be discouraged "by those who use your lawful actions as a cover for their heinous violence."

The other attacks on police included a Georgia man who authorities said called 911 to report a break-in, then ambushed the officer who came to investigate. That sparked a shootout in which both the officer and suspect were wounded but expected to survive.

In suburban St. Louis, a motorist shot an officer at least once as the officer walked back to his car during a traffic stop, police said. The officer was hospitalized in critical condition.

And in Tennessee, a man accused of shooting indiscriminately at passing cars and police on a highway told investigators he was angry about police violence against African-Americans, authorities said.

Video from the Dallas scene showed protesters marching along a downtown street about half a mile from City Hall when shots erupted and the crowd scattered, seeking cover. Officers crouched beside vehicles, armoured SWAT team vehicles arrived and a helicopter hovered overhead.

"Everyone just started running.''

Demonstrations were held in several other U.S. cities Thursday night to protest the police killings of two more black men: A Minnesota officer on Wednesday fatally shot Philando Castile while he was in a car with a woman and a child, and the shooting's aftermath was livestreamed in a widely shared Facebook video. A day earlier, Alton Sterling was shot in Louisiana after being pinned to the pavement by two white officers. That, too, was captured on a cellphone video.

The Dallas shootings occurred in an area of hotels, restaurants, businesses and some residential apartments only a few blocks from Dealey Plaza, the landmark made famous by the Kennedy assassination.

The scene was chaotic, with officers with automatic rifles on the street corners.

"Everyone just started running," Devante Odom, 21, told The Dallas Morning News. "We lost touch with two of our friends just trying to get out of there."

Carlos Harris, who lives downtown, told the newspaper that the shooters "were strategic. It was tap, tap, pause. Tap, tap, pause," he said.

“Our hearts are broken.”

Video posted on social media appeared to show a gunman at ground level exchanging fire with a police officer who was then felled.

The mayor said one of the wounded officers had a bullet go through his leg as three members of his squad were fatally shot around him.

Dallas residents stand near the scene where four Dallas police officers were shot and killed on Friday in Dallas, Texas. (Photo: Ron Jenkins/Getty Images)

"He felt that people don't understand the danger of dealing with a protest," said Rawlings, who spoke to the surviving officer. "And that's what I learned from this. We care so much about people protesting, and I think it's their rights. But how we handle it can do a lot of things. One of the things it can do is put our police officers in harm's way, and we have to be very careful about doing that."

Few details about the slain officers were immediately available.

Four of the dead were with the Dallas Police Department, a spokesman said. One was a Dallas Area Rapid Transit officer. The agency said in a statement that 43-year-old officer Brent Thompson, a newlywed whose bride also works for the police force, was the first officer killed in the line of duty since the agency formed a police department in 1989.

"Our hearts are broken," the statement said.

United States President Barack Obama delivers remarks on the police shootings in Dallas after meeting with European Union leaders at the NATO Summit in Warsaw, Poland on Friday. (Photo: Jonathan Ernst/Reuters)

Theresa Williams said one of the wounded civilians was her sister, 37-year-old Shetamia Taylor, who was shot in the right calf. She threw herself over her four sons, ages 12 to 17, when the shooting began.

Other protests across the U.S. on Thursday were peaceful, including in New York, Atlanta, Chicago and Philadelphia. In Minnesota, where Castile was shot, hundreds of protesters marched in the rain from a vigil to the governor's official residence.